These Diodes.???

[jpfamps]

That's true. And it's only one of the issues involved in coming up with a comprehensive protection scheme.

In doing any protection setup, you have to be really clear what you're protecting, and what you're willing to sacrifice (if anything) to protect parts in priority order. It's great if one solution protects everything, but you may not always be so lucky.

I think my priority here would be to protect the transformers.

Case in point: the current limiter. It's not for protecting the power transformer. It's for softening the sudden current pulse on coming out of standby on the rectifier tube, and secondarily for limiting the inrush current stress on the first filter cap. In the AC30 particularly, the current inrush coming off standby is big enough to damage the rectifier tubes, it seems.

Might have been easier to remove the standby switch.......

One issue that would definitely come up is that neither guitarists nor old-school techs are likely to appreciate some protection setup turning their amp off. The same reasoning that leads to putting aluminum foil over fuses would see the protection circuit as the problem, not the way to keep the amp from emitting black smoke. It is likely that if some hypothetical protection setup really did work and kept the amp from doing more damage to itself from loss of bias, shorted tube, shorted rectifier, etc. that the *protection circuit* would be blamed for being faulty and not letting the amp work.

This is criticism of the new SVTs, they are prone to going into shut down at the drop of a hat, especially if the mains voltage is not very well regulated, a problem a band I know had in Austin!

EDIT: In thinking about this, it might make sense to have a "don't do nothing!" switch to tell the protection circuit to just sit and watch it burn. That would make for a great - if expensive! - teaching tool.

Some people only really learn the hard way.

I would have guessed that loss of bias was more common, but that is just a guess. That condition has a lower fault current than a shorted tube.

I see shorted valves much more frequently than bias failure (yes even in the land of the JCM2000). The consequences of bias failure though are usually worse, ie dead transformers.

At this point I don't have amp designs cooking - too much else on my plate. But I can't stop myself from designing. It's a personal curse.

I've a big pile envelopes with random scribblings on too, many of which will never (and shouldn't ever) see the light of day.

[R.G.]

I think my priority here would be to protect the transformers.

Me too. I worked for some time in a Reliability/Availability/Serviceability area. The generic priority path was
1. protect the people (i.e., don't electrocute or burn them up) even if the equipment becomes junk doing so.
2. protect the data where this doesn't conflict with 1.
3. protect the money, in terms of expense to repair, where this doesn't conflict with 1 or 2. This devolved down to making a list parts and assemblies for service, sorting them by price and failure rate estimate, then devising protection schemes for most-expensive-first.

In a tube amp, we'll assume that the builder has done a good and proper job of working out 1, and there isn't data to protect. So the logical path is to list the parts inside in terms of expense to replace, and decide how far down the list to protect. PT is probably first, OT is second, and after that it may be output tubes (I had a friend just spend $200 on a pair of NOS 6L6s) or filter caps, choke, etc.

So the protection for PT and OT comes first; then you decide how to protect output tubes, given that you have to already be protecting the transformers, etc.

Might have been easier to remove the standby switch.......

That was the suggested solution. Solder a wire across it. I think that's a little abrupt. I'd probably rewire the standby switch to mute the signal into the power amp.

This is criticism of the new SVTs, they are prone to going into shut down at the drop of a hat, especially if the mains voltage is not very well regulated, a problem a band I know had in Austin!

It's difficult to do a protection scheme that's non-intrusive. Even if it perfectly limited just before damage, the users would always blame the protection, because after all, there was never any damage after we reset the protection circuits. That being said, it does take some serious thought to protect devices with as much inherent toughness as transformers and tubes without also protecting too soon.

If it was really a sensitivity to AC mains voltage on the SVTs would be to put in an AC mains ferroresonant transformer and pre-regulate the AC. Costs $50-200 depending on where you get it and removes the AC line voltage as an excuse.

I see shorted valves much more frequently than bias failure (yes even in the land of the JCM2000). The consequences of bias failure though are usually worse, ie dead transformers.

Shorted tubes being common is a sobering thought.

The neat thing about using a semiconductor "off" switch in B+ is that it is even possible to turn it off if you can sense loss of bias or a tube short. Hmm. Loss of bias leaves the tube at about 30-50V from plate to cathode, short leaves it nearly zero, so you could possibly detect the difference.

[jpfamps]

Shorted tubes being common is a sobering thought.

The neat thing about using a semiconductor "off" switch in B+ is that it is even possible to turn it off if you can sense loss of bias or a tube short. Hmm. Loss of bias leaves the tube at about 30-50V from plate to cathode, short leaves it nearly zero, so you could possibly detect the difference.

The main issue I find with shorted valves is the potential collateral damage to PCBs and the associated hassle with removing said PCB to effect a repair. Quite often the damage is self limiting as the valve goes open circuit. In fact I've seen several amps where a valve has shorted and gone open circuit, and the user replaced whatever fuses blew and simply ignored the extra hum!

Other than JCM2000s, the major causes for loss of bias are 1) leaky/ shorted filter cap in bias supply, 2) poor connection between the grid pin and valve socket (I saw a lot of this when JJ decided to reduce the size of their valve pins) 3) loss of internal connection to grid (rare but it happens).

1) can be detected by monitoring the bias voltage, 2 & 3 can't. Monitoring plate to cathode voltage would of course be a better bet here.

[Enzo]

Kinda related: We used to be a Klipsch Pro "dealer" and service center. We got a speaker cab in once with fuse holders, and in the holders were 25A fuses from the factory.

The factory explanation was that fuses do a poor job of protecting speakers. Small enough to do any good, and they blow all the time on peaks. Large eneough to stay whole, they do little or no protecting. But consumers were demanding fuses. "WHy don't you put fuses on the speakers?" SO they added fuse holders. They put the 25A fuses in them to turn them into wires, with the note that a customer could decide on a suitable value fuse and install it in their place if he wants fuse protection.

[R.G.]

I think that last is what is known in the customer-"service" biz as "plausible deniability".